As a technology for removing nitrogen oxides in exhaust gases from boilers and the like, a system of bringing the exhaust gas into contact with a catalyst to react using ammonia as a reducing agent, and thereby decomposing nitrogen oxides into harmless nitrogen and steam, now constitutes the mainstream at present. However, since ammonia used as a reducing agent is a toxic substance, handling during transport or storage of ammonia is difficult, and since unreacted ammonia causes atmospheric pollution, there is a strong demand for the development of a NOx removal technology using a reducing agent other than ammonia. Thus, extensive research is being conducted about a large number of various reducing agents.
Among these, there has been disclosed a technology of purifying exhaust gas by means of a silver-supported alumina catalyst using ethanol as a reducing agent (Non-Patent Document 1). Furthermore, there have been proposed a catalyst in which silver and sulfuric acid, or silver sulfate is supported on an inorganic oxide such as alumina (Patent Document 1), or a catalyst having optimized contents of SiO2, Na2O and chlorine in a silver-supported alumina catalyst (Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 6-198131
Patent Document 2: JP-A No. 2000-15099
Non-Patent Document 1: Applied Catalysis B: Environmental 13 (1997), p. 157-165